Show the snail icon on low power charge slots in the plan

[springfall2008]

This is an automated draft PR generated from issue #5352 — a maintainer should review it before merging.

Fixes #5352

Summary

A Chrg slot in the plan now carries the snail (🐌) that a slow export slot already shows, when low power charging (switch.predbat_set_charge_low_power) is asking for less than the maximum charge rate in that slot.

Export stores its reduced rate in the limit, so the existing check is a simple decode. Charge has no such encoding - the rate is worked out by find_charge_rate() as the slot runs - so Output.get_charge_rate_kw(), which already recomputes that rate for the slot's description text, now also returns whether the requested rate is below battery_rate_max_charge. publish_html_plan() appends the snail on that flag.

docs/predbat-plan-card.md already promised the snail for a slow charge; the sentence is reworded to match what the code does (the snail follows the State, and the limit.tens_of_percentage_rate_reduction form is export-only and only shown with plan debug on).

Testing

• ./run_pre_commit (from coverage/): all hooks passed.
• tools/triage_test.sh plan_why_reason: fails without the fix, passes with it. With output.py stashed the run fails on the new assertion only (Chrg low power state cell should show the snail like a slow export does (#5352), got: Chrg↗); with the fix restored it passes.
• New assertions in test_plan_why_reason.py: the throttled low power slot's state_html is exactly Chrg↗🐌; a full rate charge with low power off shows no snail; and low power on but a target too far away to throttle (so find_charge_rate() falls back to the maximum rate) shows no snail. The two no-snail cases pass either way by construction - they are there to pin the behaviour, the first is the one that proves the change.
• tools/triage_test.sh debug_cases: passed (golden plan JSON unchanged).

Notes

• What counts as slow. The snail keys on the rate low power mode requests, not the rate achieved after the battery charge curve. A full rate charge that the curve tapers near the top of the battery therefore gets no snail, which mirrors export, where the snail marks a planned reduced rate rather than a physical limit. The issue says "when the power is less than the maximum"; if the taper should count too, the comparison in get_charge_rate_kw() is the one line to change.
• Per-row, from predicted SoC. Like the rate_kw in the description text, the flag is recomputed for each plan row from the predicted SoC at the start of that row, so a long charge window can show the snail on some rows and not others (e.g. once the remaining window is too short to throttle).
• get_charge_rate_kw() keeps its name but now returns a (rate_kw, low_power) tuple. GitNexus impact() reports LOW risk: one caller, publish_html_plan(), updated here.
• The snail is appended to the same state string as the export one, so it flows into state_html / state_text in the plan JSON the same way; state (raw Chrg) is unchanged.

🤖 Generated with Claude Code

[Copilot]

🟡 Changes recommended

Partial-row charge windows can be incorrectly marked as low power because rate calculation begins before the charge window.

1 open finding

• Clamp charge-rate recomputation to the actual charge-window start · New

What changed in this PR

Adds a snail indicator to plan rows where low-power charging requests a reduced rate.

Changes:

• Returns charge rate and low-power status from get_charge_rate_kw().
• Displays and tests the snail indicator.
• Clarifies plan-card documentation.

File	Description
apps/​predbat/​output.py	Calculates and displays reduced-charge status.
apps/​predbat/​tests/​test_plan_why_reason.py	Adds charge-rate and snail regression cases.
docs/​predbat-plan-card.md	Documents slow-charge and slow-export indicators.

🧠 Review effort: Balanced

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